Low metalization creep sensor

ABSTRACT

Performance-enhancing, reduced-area metalization adhesion areas in force-sensing transducers are described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority from U.S. patentapplication Ser. No. 09/104,844 filed Jun. 25, 1998 now U.S. Pat. No.6,161,440, which claims priority from U.S. Provisional Application No.60/055,646, filed Aug. 14, 1997, and incorporated by reference herein.

TECHNICAL FIELD

The present invention relates generally to force-sensing transducers,accelerometers, rate sensors, methods of forming force-sensingtransducers, and to methods of forming vibrating-beam force transducers.

BACKGROUND OF THE INVENTION

Force-sensing transducers can be used to measure force, acceleration,pressure, and the like. One type of force-sensing transducer is aresonating force transducer. Exemplary transducers are described in U.S.Pat. Nos. 5,367,217, 5,339,698, and 5,331,242, the disclosures of whichare incorporated by reference. Another type of force-sensing transduceris an accelerometer. Exemplary accelerometers are described in U.S. Pat.Nos. 5,594,170, 5,501,103, 5,379,639, 5,377,545, 5,456,111, 5,456,110,and 5,005,413, which are incorporated by reference herein. Other typesof force-sensing transducers can be used as rate sensors. Exemplary ratesensors are described in U.S. Pat. Nos. 5,717,140, 5,376,217, 5,696,323,5,691,472, and 5,668,329, which are hereby incorporated by reference.Yet other force-sensing transducers can be used as acceleration and ratesensors. Exemplary sensors are described in U.S. Pat. Nos. 5,627,314,5,557,046, 5,341,682, 5,331,853, 5,331,854, and 5,319,976, thedisclosures of which are incorporated by reference herein.

Force-sensing transducers such as those incorporated by reference above,can experience problems associated with metalization which can adverselyaffect the transducer's performance. In particular, bias performance canbe adversely affected when a conductor having a thermal coefficient ofexpansion different from that of the substrate, is deposited and usedduring operation. Specifically, metal conductors can be deposited athigh temperatures and, because of the difference in thermal coefficientof expansion with the substrate, the deposited metal conductor can“creep” because of high thermal stress developed between the metalconductor and the substrate. Metal creep occurs when the deposited metalyields during the application of some external stimulant and does notreturn to its initial condition. The change in condition results in achange in bias operating point.

A preferred conductor material is gold. Gold is typically used informing metal conductors on such substrates because it exhibits highconductivity and other traits normally associated with noble metals.However, forming the metal conductors using gold exacerbates the adverseeffects on bias performance because gold has a very high thermalexpansion coefficient relative to typical substrate materials such asquartz and silicon. Additionally, gold has a very low yield strength.

Traditional methods of reducing metal creep include using metal alloyswith higher yield strength than pure gold, using alloys with thermalcoefficients of expansion closely matched to the particular substratematerial, removing metalization layers all together, compensating themetal creep effects by matching metal conductors on opposing surfaces,and designing a very low spring rate support structure to counter theeffects of creep.

This invention arose out of concerns associated with providing improvedforce-sensing transducers, accelerometers, and rate sensors. Thisinvention also arose out of concerns associated with providing improvedmethods of forming force-sensing transducers such as those mentionedabove.

SUMMARY OF THE INVENTION

Force-sensing transducers, accelerometers, rate sensors, and methods offorming force-sensing transducers are described.

In one embodiment, a substrate includes a force-sensing element. Anadhesion layer is disposed over less than an entirety of theforce-sensing element, and a conductive layer is disposed over theforce-sensing element and supported in a bonded relationship therewiththrough the adhesion layer.

In another embodiment, a substrate includes a proof mass and avibratable assembly connected therewith and configured to detect anacceleration force. An adhesion layer is disposed over less than anentirety of the vibratable assembly, and a conductive path is disposedover the vibratable assembly and fixedly bonded therewith through theadhesion layer.

In another embodiment, a Coriolis rate sensor includes a substratehaving a vibratable assembly connected therewith. An adhesion layer isdisposed over less than an entirety of the vibratable assembly and aconductive path is disposed over the vibratable assembly and fixedlybonded therewith through the adhesion layer.

In yet another embodiment, a substrate is provided having aforce-sensing element defining an area over and within which aconductive layer of material is to be formed. A patterned adhesion layeris formed over less than an entirety of the area. A conductive layer isformed over the area and bonded to the substrate through the patternedadhesion layer.

In still another embodiment, a substrate is provided and etchedsufficiently to form a plurality of vibratable beams arranged in aforce-sensing configuration. An insulative layer of material is formedover the vibratable beams and an adhesion layer pattern is formed overthe vibratable beams. The pattern comprises a plurality of spaced-apartpattern components, with each beam having three pattern componentsspaced apart along its length. Conductive material is formed over thevibratable beams and the adhesion layer pattern. The conductive materialis more fixedly attached to the adhesion layer pattern than tovibratable beam portions not having the adhesion layer patternthereover. The substrate is temperature cycled effective to weaken theattachment between the conductive material and the vibratable beamportions not having the adhesion layer pattern thereover.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a force-sensing transducer which can beutilized in connection with one or more embodiments of the invention.

FIG. 2 is a top plan view of a different force-sensing transducer whichcan be utilized in connection with one or more embodiments of thepresent invention.

FIG. 3 is a top plan view of an accelerometer which can be utilized inconnection with one or more embodiments of the present invention.

FIG. 4 is a top plan view of an acceleration and rate sensor which canbe utilized in connection with one or more embodiments of the presentinvention.

FIG. 5 is a top plan view of a force-sensing transducer in accordancewith one embodiment of the present invention.

FIG. 6 is an exaggerated elevational view of a vibrating beam clamped onboth ends, and shown in an excited state.

FIG. 7 is a view of the FIG. 5 force-sensing transducer at a processingstep which is subsequent to that which is shown in FIG. 5.

FIG. 8 is a view which is taken along line 8—8 in FIG. 7, and shows aportion of the illustrated force-sensing transducer at a processing stepwhich is subsequent to that which is shown in FIG. 7.

FIG. 9 is a view of the FIG. 8 force-sensing transducer portion at aprocessing step which is subsequent to that which is shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-4 show various embodiments of force-sensing transducers whichcan be utilized in connection with one or more embodiments of theinvention described below.

FIG. 1 shows a force-sensing transducer generally at 10 which can beused in connection with one or more embodiments of the inventiondescribed below. Transducer 10 includes a substrate 12 having aforce-sensing element 14 which is preferably etched from the substratematerial. In the illustrated and preferred embodiment, force-sensingelement 14 comprises a vibratable assembly having a plurality ofvibratable beams 16, 18, 20, and 22. Of course, more or less beams canbe provided. In a preferred embodiment, the force-sensing elementcomprises an integral material, with crystalline silicon beingpreferred. The specific force-sensing transducer shown in FIG. 1 isdescribed in more detail in U.S. Pat. No. 5,367,217, incorporated byreference above.

FIG. 2 shows a different force-sensing transducer generally at 10 a.Like numerals from the above-described embodiment have been utilizedwhere appropriate, with differences being indicated by the suffix “a” orwith different numerals. Force-sensing transducer 10 a is configured asan angular rate sensor or Coriolis rate sensor. Vibratable assembly 14 aincludes a plurality of vibrating beams 24, 26, 28, and 30 defining anouter pair of vibrating beams 24, 28, and an inner pair of vibratingbeams 26, 30. Outer beams 24, 28 are mechanically coupled to the innerbeams 26, 30 by way of interconnecting members 32, 34. Interconnectingmembers 32, 34 are formed generally at the midpoint of outer beams 24,28 and inner beams 26, 30. Beams 24, 26, 28, and 30, as well asinterconnecting members 32, 34 are formed by forming slots (notspecifically designated) between the beams. The operation offorce-sensing transducer 10 a is described in detail in U.S. Pat. No.5,717,140 incorporated by reference above.

FIG. 3 shows a force-sensing transducer in the form of an accelerometergenerally at 10 b, which can be configured in accordance with variousembodiments of the present invention. Like numerals from theabove-described embodiments are utilized where appropriate, withdifferences being indicated by the suffix “b” or with differentnumerals.

Accelerometer 10 b is preferably formed from a single wafer of siliconthrough micromachining techniques. Accelerometers such as this andothers are described in U.S. Pat. No. 5,005,413, which is incorporatedby reference above. Accelerometer 10 b is configured as a force-sensingtransducer which senses an acceleration force which acts upon theaccelerometer. In this example, substrate 12 b includes a proof mass 36and a vibratable assembly 14 b connected therewith and configured todetect the acceleration force. Proof mass 36 is supported by a pair offlexures 38, 40. In this example, force-sensing transducer 10 bcomprises a vibrating beam accelerometer whose vibratable assembly 14 bcomprises a plurality of vibratable beams 42, 44 and 46, 48. The beamsare arranged, together with proof mass 36, in a configuration whichdevelops different vibratory frequencies responsive to movement ordeflection of proof mass 36 in accordance with an acceleration appliedthereto. The various frequencies at which the beams vibrate give anindication of the acceleration force acting upon the proof mass. Theacceleration-sensing operation of this force-sensing transducer isdescribed in U.S. Pat. No. 5,379,639 incorporated by reference above.

Briefly, vibratable assembly 14 b is configured into a push-pullconfiguration such that when proof mass 36 experiences an acceleration,it is moved or deflected along an input axis either generally into orout of the plane of the page upon which FIG. 1 appears. Such movement ordeflection causes one of the pairs of beams 42, 44 or 46, 48 to go intotension, and the other of the pairs of beams to go into compression. Asthis compression and tension occurs, the respective frequencies at whichthe beams vibrate change in a known and measurable manner.

FIG. 4 shows a force-sensing transducer 10 c in the form of anacceleration and angular rate sensing device. In the illustratedexample, the device includes a first accelerometer 50 having a proofmass 52 connected to substrate 12 c by a pair of flexures 54. Theflexures define a hinge axis HA₁ for proof mass 52. A secondaccelerometer 56 is provided and includes a proof mass 58 and a pair offlexures 60 connecting the proof mass to substrate 12 c. Flexures 60define a second hinge axis HA₂ for proof mass 58. A pair offorce-sensing elements 14 c are provided in the form of a vibratableassembly similar to the one described below in FIG. 5. A link 62 isprovided and connects accelerometers 50, 56 together and acts as arocker arm to ensure that the accelerometers can be dithered 180° out ofphase. A discussion of various aspects of the dynamics of dithering assuch pertains to accelerometers and rate sensors can be found in many ofthe references above. The specific structure and operation of the FIG. 4acceleration and rate sensor is described in U.S. patent applicationSer. No. 08/949,883, the disclosure of which is incorporated byreference herein.

A plurality of contact pads T₁, G₁, R₁, P₁, D₁, and D₂, P₂, R₂, G₂, andT₂, are disposed over the substrate and provide contacts through whichdesirable electrical connection can be made to outside-world circuitryfor the dither drive, dither pick-off, beam or tine drives, and beam ortine pick-offs. Conductive material, such as gold, is disposed over thecontact pads and various other areas of the substrate includingvibratable assemblies 14 c.

In the devices described above, each uses a vibratable assembly to sensea force such as acceleration or angular rate. Aspects of the inventiondescribed just below are well suited for implementation in connectionwith these and other devices.

For illustrative purposes only, an exemplary force-sensing transducer isshown in FIG. 5 generally at 10 d. Like numerals from theabove-described embodiment have been utilized where appropriate, withdifferences being indicated with the suffix “d” or with differentnumerals. A force-sensing element or vibratable assembly 14 d isprovided and includes a plurality of beams 64, 66 which are preferablyvibratable. A non-vibratable assembly 68 is provided for supporting areference resistor which will ultimately be formed when conductivematerial is provided over the vibratable and non-vibratable assemblies14 d, 68 respectively. Non-vibratable assembly 68 includes a pair ofbeams 70, 72 which are tied together by a plurality of pins 74, 76, and78. Beams 70, 72 do not vibrate and are provided to match the electricalpath resistance of beams 64, 66 when conductive material is disposedthereover. When an electrical current is provided over a conductive pathover vibrating beams 64, 66, a voltage drop develops relative to thosebeams. Similarly, a current provided over beams 70, 72 develops agenerally identical voltage drop such that the two voltage drops can besubtracted from one another and cancel. This assists outside circuitryin its discrimination of the feedback signals from the vibrating beams.Support structures 80 are provided and tie the ends of both thevibratable and non-vibratable beams together. Beams 64, 66, 70, and 72extend along respective long axes and define individual respective areasover and within which a conductive layer of material is to be formed.

FIG. 6 shows an exemplary schematic of a beam B with neutral inflectionpoints P₁ and P₂. A discussion of neutral inflection points occurs inU.S. Pat. No. 5,717,140 incorporated by reference above.

Referring to FIGS. 7 and 8, an insulative material layer 82 is formedover the beam assemblies. An exemplary material is silicon dioxide whichcan be grown to a thickness of 5,000 Angstrom. An adhesion layer pattern84 is formed over substrate 12 d. In the illustrated example, adhesionlayer pattern 84 is formed over both the vibratable assembly 14 d andthe non-vibratable assembly 68. In addition, the adhesion layer patternis formed over support structures 80 as shown. The adhesion layerpattern is preferably disposed over less than an entirety offorce-sensing element or vibratable assembly 14 d. In one embodiment,adhesion layer pattern 84 comprises a plurality of discrete layers whichare spaced apart over vibratable assembly 14 d. In the illustratedexample, each beam 64, 66, 70, and 72 includes three discrete layers ofadhesion material. Accordingly, the individual discrete layers define aplurality of spaced-apart pattern components which are disposed alongeach beam's length.

In a preferred embodiment, each vibratable beam has at least one neutralinflection point such as P₁ and P₂ (FIG. 6) and one of the discreteadhesion layers is disposed over the one neutral inflection point. Inthe illustrated example, beams 64 and 66 have two neutral inflectionpoints which are disposed on either side of the beam's centermost point.As viewed in FIG. 8, the leftmost and rightmost adhesion layer patterns84 are disposed over the neutral inflection points of beam 64.Accordingly, each vibratable beam has at least one neutral inflectionpoint having a discrete adhesion layer disposed thereover. In a mostpreferred embodiment, each beam has a discrete layer disposed over itscentermost point, as well as each neutral inflection point. In thisexample, each beam has no more than three discrete adhesion layersdisposed thereover and spaced therealong.

A preferred material for the adhesion layer pattern is chrome which canbe patterned onto the beams through shadow masking techniques. Othermanners of providing a patterned adhesion layer can be used. Anexemplary thickness for the patterned adhesion layer is 100 Angstrom. Inthis example, the preferred chrome is deposited at both ends of avibrating beam in 67-micron-wide sections. Limiting the chromedeposition to support structures 80 and the above-described threespaced-apart locations, reduces the total area of chrome deposition fromabout 1,000 microns to about 200 microns which results in a 5-to-1reduction in the adhesion area.

Accordingly, application of the present invention to a vibrating beamprovides advantages which help to reduce metal creep. By defining thechrome deposition areas at the neutral bending or inflection points,e.g. approximately 22.4% of the length from where the beam attaches tosupport structures 80, the stress in the conductor, in this case gold,is further reduced. Defining a chrome deposition area at the midpoint ofthe beam tends to stress the deposited metal less than elsewhere, thoughthe midpoint is not at a neutral inflection point. Thus, the gold isbonded to the beam through the chrome interface at three points alongthe length of the conductive path, as well as at both ends. The numberof points of attachment and the area of adhesion can be varied as thoseof skill in the art will realize, by reducing the attachment width toachieve a similar overall reduction in the attachment area. Similarly,the pattern may be varied along the width of the beam as well as alongits length, for example, in a checkerboard pattern. While there areinnumerable numbers of pattern variations, the spirit of the inventionremains the same in reducing metal creep effects by reducing the totaladhesion area.

FIG. 8 shows a conductive material layer or path 86 formed overvibratable beam 64 and the adhesion layer pattern 84. Conductive layer86 is more fixedly attached to the adhesion layer pattern than tovibratable beam portions 88 which do not have adhesion layer pattern 84thereover.

FIG. 9 shows vibrating beam 64 after the beam has been temperaturecycled to effectively weaken the attachment between conductive material86 and vibratable beam portions 88. By temperature cycling the finishedbeam over a greater range of temperatures than expected during actualuse, for example temperature cycling from −65° C. to 135° C. for usefrom −55° C. to 125° C., the conductive material 86, e.g. gold-to-oxide,adhesion bond is broken in the oxide area but the conductivematerial-to-chrome bond remains intact. Particularly, the excess coldcycle stresses the gold past its yield point which helps activate thereduction in metal creep. Such temperature cycling causes a portion ofconductive layer 86 which is not bonded with the patterned adhesionlayer to separate from the underlying substrate material.

Some of the advantages achieved by the present invention are thatadverse effects of metalization on bias performance in force transducerscan be reduced by providing a large reduction in metal creep while usingstandard conductors such as gold. Reductions in metal creep are achievedthrough the provision of reduced metalization adhesion areas. Invibrating beam force sensors, additional bias performance enhancementcan be provided by forming the metalization adhesion areas at a beam'sneutral inflection point(s). Such advantages can be achieved withoutmeaningful increases in cost by using traditional metalizationmaterials, for example, gold, and by taking advantage of readilyavailable supplies, masks, and processing procedures.

The invention has been described in compliance with the applicablestatutes. Variations and modifications will be readily apparent to thoseof skill in the art. It is therefore to be understood that the inventionis not limited to the specific features shown and described, since thedisclosure comprises preferred forms of putting the invention intoeffect. The invention is, therefore, to be interpreted in light of theappended claims appropriately interpreted in accordance with thedoctrine of equivalents.

What is claimed is:
 1. An accelerometer comprising: a substrate having aproof mass and a vibratable assembly connected therewith, the proof massand the vibratable assembly being configured to detect an accelerationforce; an adhesive layer disposed over less than an entirety of thevibratable assembly; and a conductive path disposed over the vibratableassembly and intermittently supported in a bonded relationship therewiththrough the adhesive layer, said bonded relationship consisting of areduced bonding area that is less than the area of said conductive path.2. The accelerometer of claim 1, wherein the adhesion layer comprises aplurality of discrete layers spaced apart over the vibratable assembly.3. The accelerometer of claim 1, wherein the vibratable assemblycomprises a vibratable beam, and the adhesion layer comprises aplurality of discrete layers spaced apart over the vibratable beam. 4.The accelerometer of claim 3, wherein the adhesion layer comprises threediscrete layers.
 5. The accelerometer of claim 3, wherein the vibratablebeam has at least one neutral inflection point, and one of the discretelayers is disposed over the one neutral inflection point.
 6. Theaccelerometer of claim 1, wherein the vibratable assembly comprises aplurality of vibratable beams, and the adhesion layer comprises aplurality of discrete layers spaced apart over individual beams.
 7. Theaccelerometer of claim 6, wherein each beam has a plurality of discretelayers spaced thereover.
 8. The accelerometer of claim 7, wherein eachbeam has at least one neutral inflection point having a discrete layerdisposed thereover.
 9. The accelerometer of claim 7, wherein each beamhas a discrete layer disposed over said beam's centermost point.